The phenomenon of dynamic gain tilt, particularly that exhibited by erbium doped optical amplifiers, presents problems in the transmission of wavelength division mutiplexed signals along a common highway that contains such amplifiers. In such a transmission system it is desirable for the channel powers to be substantially balanced, otherwise, in respect of the strong channels, there are liable to be problems associated With non-linearity effects, while in respect of the weak channels, there are liable to be problems associated with poor signal to noise ratio. In systems of the future, the introduction of optical drop/insert and optical routing/crossconnects will lead to much greater problems of power imbalances between channels.
One attempt at a solution to this problem is described in the specification of U.S. Pat. No.5,452,116. This involves the use of spatially separated amplifiers at intervals along the highway where the different channels are demultiplexed, separately amplified, and remultiplexed again. This approach becomes increasingly unattractive to implement as the number of channels is increased. Additionally it relies upon using preset wavelength filters to break up the full WDM band into independent regions. This inevitably means that there must be spectral deadbands between adjacent regions, which is wasteful of available spectrum. Some degree of alleviation of the problem is alternatively achieved by the use of multi-core erbium fibre so as to obtain some flattening of the erbium gain characteristic. This is for instance described by M Zervas et al. in the paper entitled, xe2x80x98Twin-Core Fiber Erbium-Doped Channel Equalizerxe2x80x99, Journal of Lightwave Technology, Vol. 13, No. 5, May 1995, pages 721-731. A further alternative is to cool the erbium fibre to give some independence of gain and saturation across the band, as reported by E Goldstein et al. in the paper entitled, xe2x80x98Inhomogeneously Broadened Fiber-Amplifier Cascades for Transparent Multiwavelength Lightwave Networksxe2x80x99, Journal of Lightwave Technology, Vol. 13, No. 5, May 1995, pages 782-790. However the effect is of limited practical scope, and cooling of this sort is undesirable additional expense.
It is an object of the present invention to provide a component for use in a WDM environment which limits the power output from the component of individual channels. It is to be noted that this is not the same as wanting constant optical gain across the full WDM band. Additionally it is an object that the component shall allow the channels to be located anywhere in the full WDM band, rather than to be confined within specific predetermined windows in the band. In essence this means that the incoming signal shall define the channel whose output power is to be limited.
According to the present invention there is provided an optically amplified optical transmission system in which a plurality of optical channels are caused to propagate in wavelength division multiplexed form along a common transmission path, which transmission path includes photorefractive reflection grating generation means adapted, in response to receipt of power in any of said channels in excess of a predetermined power limit, to create and sustain, for the duration of said receipt of power, a Bragg reflection grating having a bandwidth extending the full spectral width of the channel, whereby a power-per-channel saturation limit is applied to the transmission path.
One way of generating the required photorefractive reflection gratings is by means of stimulated Brillouin scattering (SBS). An alternative way relies upon creating a grating through the agency of the Kerr effect, and another through creating a thermal grating, for instance in a liquid crystal medium.
The invention also provides, in an optically amplified optical transmission system in which a plurality of optical channels are caused to propagate in wavelength division multiplexed form along a common transmission path, a method of applying a saturation power limit to the optical power transmitted through said transmission path by using optical power launched into said transmission path to control the reflectance of a dynamic reflectance reflector located in said transmission path.
According to a further aspect of the invention there is provided a method of balancing channel power in a wavelength division multiplexed optically amplified transmission system, said system having a transmission path carrying a plurality of wavelength division multiplexed channels, said system including dynamic reflectance reflection grating generation means, said method comprising generating the dynamic reflectance reflection grating in the system in response to an out-of-balance power level in a channel, and balancing, by means of said generated grating, the power in that channel towards the power per channel of other channels propagating in said transmission path.